1. Field of the Invention
This invention relates to an actuator for a valve and, more specifically, to such an actuator which includes a hydraulic cylinder and piston arrangement in which a generally sperical shaped accumulator is mounted on the cylinder to provide a reservoir of high pressure gas for reliable closure of the valve as hydraulic pressure is relieved.
2. Description of the Prior Art
There have herebefore been utilized a number of means for closing large valves in the feedwater and steam systems of nuclear power plants. Because of safety and reliability requirements for these plants, the time required to close the valves within the system and the ability of the valves to safely withstand seismic shock are of real concern. While electrical motor operators have been used for reliable closure of heavy duty valves of this type in the past, the closure time required has generally made them unattractive.
It is presently felt that a valve actuator incorporating a hydraulic cylinder and gas accumulator affords the best means for rapid closure of the valve. Hydraulic oil is directed to a piston within the hydraulic cylinder to open the valve in opposition to a precharged source of high pressure gas which acts on the other side of the piston. A reservoir of the high pressure gas is maintained in an accumulator to ensure a sufficient quantity is available to act on the piston to rapidly close the valve when the hydraulic pressure is relieved. While it is obvious that an accumulator might be provided in the form of a tank at a remote location from the valve, it is preferred to have a self-contained system. One such system presently employed for use with balanced globe valves up to 32" includes a cylinder within a cylinder configuration in which the high pressure gas is located between the cylinders. However, this configuration is not inexpensive to provide and the size and weight of the outer cylinder required to provide ample closing forces is recognized as being a significant factor to be considered in a determination of the ability of the valve to withstand seismic shock.
Additionally, although this configuration has ensured rapid closure of balanced valves in the past, its applicability for use with unbalanced gate valves is questionable. There are indications that the larger forces which would be needed to close a comparable sized gate valve would require an outer cylinder of significantly larger dimensions and weight, greatly complicating the seismic problems and space problems which must be considered in power plant design.